Season: 2

An amnesiac Olivia makes her return from the parallel universe, while Walter, Pete, and Junior Agent Amy Jessup investigate the mysterious death of a man found with puncture holes in his palate. Meanwhile, Broyles defends the Fringe Division from a Senate subcommittee threatening to shut the organization down.

Walter explains to Olivia how Peter came from the alternate universe, and how he was willing to risk everything to save his son.

The arrival of a building from the alternate Earth causes massive death and destruction to those occupying the site at its arrival. Walter soon realizes that to equalize the balance between universes, another building will have to disappear… along with everyone inside it. The team's only chance is for Walter to activate Olivia's long-buried ability to distinguish objects from the other universe.

When all of the groom's relatives asphyxiate at a wedding, the Fringe division investigates and quickly ascertain that a human agency is responsible. Walter realizes that the formula for the toxin was based on research conducted by his father, Robert Bischoff, and that the killer isn't stopping with a single test of his lethal concoction.